making oncall ical events transparent (#327)

so that they don't show up as busy on people's calendar
Reference: https://www.kanzaki.com/docs/ical/transp.html

src/oncall/api/v0/ical.py

@@ -60,6 +60,7 @@ def events_to_ical(events, identifier, contact=True):
         cal_event.add('description',
                       '%s\n' % full_name +
                       ('\n'.join(['%s: %s' % (mode, dest) for mode, dest in user['contacts'].items()]) if contact else ''))
+        cal_event.add('TRANSP', 'TRANSPARENT')
 
         # Attach info about the user oncall
         attendee = vCalAddress('MAILTO:%s' % (user['contacts'].get('email') if contact else ''))